Most likely you have one of the older, full-height adapters that are not compatible with newer PCI Express slots. Check for a product code on the adapter. The older, full-height adapters had these codes:
If your adapter has one of the product codes listed above, you will not be able to get the adapter to work.
Newer versions of the adapter called the Intel® PRO/1000 PT Quad Port Low Profile Server Adapter with these codes do not have the incompatibility issue with newer PCI Express slots:
When I looked at the Server Configurator Tool, I did not see the older, full-height adapter listed, so the most likely solution is to use a newer network adapter.